1. Field of the Invention
The present invention generally relates to liquid crystal display devices and, more particularly, is directed to a liquid crystal display with back light.
2. Description of the Prior Art
A variety of liquid crystal displays (hereinafter simply referred to as LCDs) have been proposed so far and a twisted nematic display mode (TN mode) liquid crystal is known as one of the most popular LCDs.
As shown in a schematic diagram forming FIG. 1, this type of LCD is comprised of X-axis and Y-axis transparent electrodes 6, 8 formed on the inner surface of a pair of glass substrates 5, 9 in the direction perpendicular to each other in a matrix fashion, a TN liquid crystal 7 sandwiched between the two electrodes 6 and 8 with a twisted orientation of 90 degrees and a pair of polarizing plates 4, 10 unitarily formed with the outer surfaces of the glass substrates 5, 9 in the direction perpendicular to each other. In this case, the pair of polarizing plates 4, 10 are bonded to the outer surfaces of the glass substrates 5, 9. A voltage is applied between the transparent electrodes 6, 8 of a liquid crystal panel 13 formed by the above-mentioned elements by means of a driving source 11 and a switching device 12. When this TN mode liquid crystal is in its off state, that is, without application of voltage, a linearly-polarized light is rotated 90 degrees and passed by the liquid crystal panel 13. When on the other hand this TN mode liquid crystal is applied with voltage and turned on, then the twisted state is removed and the linearly-polarized light is inhibited from passing the liquid crystal panel 13.
When the LCD is constructed by using such liquid crystal panel 13, the LCD is generally formed as a reflection type, a reflection type using a back light or a transparent type. The kinds of back light 2 are selected in accordance with the purpose that for which the LCD is to be used. The back light might be an incandescent lamp, a fluorescent lamp, an electroluminescent lamp (EL) and so on. The fluorescent lamp is a light source suitable for color LCD and widely used because the fluorescent lamp produces light having a plurality of peaks of brightness in the visible region and this light becomes substantially white light. The fluorescent lamp is generally formed as a hot cathode type or a cold cathode type. The hot cathode type of fluorescent lamp is driven by a voltage of from 200 to 300 Volts and the cold cathode type of fluorescent lamp is driven by a high voltage of nearly 4000 Volts. The back light 2 formed of, for example, the fluorescent lamp is housed in a casing 1 having a diffuser 3 on the front surface thereof. This casing 1 is generally made of metal and the diffuser 3 is made of a white plastic plate or the like. The casing 1 has the liquid crystal panel 13 unitarily assembled into the front portion of the diffuser 3 as shown in FIG. 2.
When the back light 2 of the LCD is driven, a voltage of 200 to 300 Volts is applied to the cathode of the hot cathode type fluorescent lamp and a high voltage of about 4000 Volts is applied to the cathode of the cold cathode type fluorescent lamp. Further, a driving source is not a commercially available voltage source but a high frequency of about 40 kHz is employed as a switching means to thereby increase light emission efficiency.
Let us now consider that such LCD is installed, for example, on the rear surface of each of the passenger seats in the cabin of an airplane so that the passengers can enjoy watching video programs of different channels. In that case, the airplane has very strict specification on the leaked electromagnetic noise so as to prevent the automatic pilot system of the airplane from being affected thereby. Particularly, since the electromagnetic noise from the fluorescent lamp 2 is emitted from the LCD panel surface through the diffuser 3 and the liquid crystal panel 13, it is necessary to provide a countermeasure to prevent the electromagnetic noise, i.e., electromagnetic waves, from being leaked. As one of the methods for preventing electromagnetic waves from being leaked, it is proposed that, as shown in FIG. 2, a conductive film 14 such as an ITO film or the like is bonded to or coated on the front surface of the liquid crystal panel 13 to thereby shield the electromagnetic noise from the back light 2. In this arrangement, however, in order to visually confirm whether the light is passed through or cut off by the liquid crystal panel 13, the conductive film 14 must be made transparent, which unavoidably decreases the light utilizing ratio of the liquid crystal panel 13.